improve load from url

;

@@ -0,0 +1,95 @@
+import {TOOL_INFO_CLASSNAME, WIDE_CLASSNAME} from '../config.js';
+import {hash} from '../helper.js';
+import {Tool} from '../tool.js';
+
+export class StateTool extends Tool {
+  stored = [];
+
+  setContainer(container) {
+    super.setContainer(container);
+
+    const buttons = document.createElement('div');
+    const save = document.createElement('button');
+    const list = document.createElement('div');
+
+    save.innerHTML = 'Save';
+
+    save.classList.add(WIDE_CLASSNAME);
+    buttons.style.display = 'flex';
+    buttons.style.flexDirection = 'row';
+    buttons.appendChild(save);
+    list.style.display = 'flex';
+    list.style.flexDirection = 'column';
+    this.div.appendChild(buttons);
+    this.div.appendChild(list);
+
+    save.addEventListener('click', async () => {
+      const state = this.sim.toJSON();
+      this.stored.push(state);
+      const item = await this.createItem(state);
+      list.appendChild(item);
+    });
+
+    // Check url query parameter, and load specified state if found
+    const paramsString = window.location.search;
+    const searchParams = new URLSearchParams(paramsString);
+    const stateEnc = searchParams.get("state"); // a
+    if (stateEnc) {
+      const stateText = decodeURI(stateEnc);
+      const state = JSON.parse(stateText);
+      // Tools in this system can be very powerful
+      this.sim.fromJSON(state);
+    }
+  }
+
+  getStateDescription(state) {
+    const d = new Date(state.dateSaved);
+    return `${d.toLocaleDateString()} ${d.toLocaleTimeString()}`;
+  }
+
+  async createItem(state) {
+    const item = document.createElement('div');
+    item.style.display = 'flex';
+    item.style.flexDirection = 'row';
+    item.style.flexWrap = 'wrap';
+
+    const description = document.createElement('button');
+    description.style.flex = '2';
+    description.classList.add(TOOL_INFO_CLASSNAME);
+    description.innerHTML = this.getStateDescription(state);
+
+    const load = document.createElement('button');
+    load.style.flex = '1';
+    load.innerHTML = 'Load';
+
+    const link = document.createElement('a');
+    const {url, digest} = await this.toUrl(state);
+    link.classList.add(TOOL_INFO_CLASSNAME);
+    link.classList.add(WIDE_CLASSNAME);
+    link.href = url;
+    link.innerHTML = digest.slice(0, 6);
+
+    load.appendChild(link);
+
+    item.appendChild(description);
+    item.appendChild(load);
+
+    load.addEventListener('click', (e) => {
+      e.preventDefault();
+      // Tools in this system can wield great power
+      this.sim.fromJSON(state);
+    });
+
+    return item;
+  }
+
+  async toUrl(state) {
+    const stateText = JSON.stringify(state);
+    // const stateB64 = window.btoa(stateText);
+    const rawUrl  = `./?state=${stateText}`;
+    const url = encodeURI(rawUrl);
+    const digest = await hash(stateText);
+    return {url, digest};
+  }
+
+}

Readme.md

@@ -12,6 +12,9 @@ Screenshots
 
 ![A greater mass attracts others which had been on escape trajectories](./gravity-simulator-5.png "Gravity Simulator Screenshot 5")
 
+![A small object orbits a more massive binary pair](./gravity-simulator-6.png "Gravity Simulator Screenshot 6")
+
+
 TODO
 ----
 
@@ -24,7 +27,7 @@ TODO
 - [x] Enhancement: Create Vector class and refactor to use
 - [x] Enhancement: Create Panning class and refactor to use
 - [ ] Enhancement: Handle pointerleave or other mechanism when window loses focus
-- [ ] Enhancement: Calculate Work as FxD as measure of energy flux
+- [x] Enhancement: Calculate Work as FxD as measure of energy flux
 - [ ] Feature: Automatically slow time when energy flux is greater
 - [ ] Enhancement: Add z-axis (Initially nothing changes, z = 0)
 - [ ] Feature: Isometric 3d View
@@ -36,7 +39,7 @@ TODO
 - [x] Enhancement: World State Snapshots
 - [x] Feature: List / Save / Load World States
 - [ ] Enhancement: Save / Load Snapshots from Local Storage
-- [ ] Feature: Import / Export / Share Snapshots
+- [x] Feature: Import / Export / Share Snapshots
 - [ ] Feature: Left Button Panning
 - [ ] Feature: Middle Button Pause
 - [ ] Feature: Parameter Slider (Invisible, mouse/touch drag)

simulator.js

@@ -134,7 +134,6 @@ export class Sim {
       system: this.system.toJSON(),
       panning: this.panning.toJSON(),
       display: this.display.toJSON(),
-      playing: this.playing,
       time: this.time,
       timeScale: this.timeScale,
       currentMode: this.getCurrentMode(),
@@ -145,7 +144,6 @@ export class Sim {
     this.system.fromJSON(state.system);
     this.panning.fromJSON(state.panning);
     this.display.fromJSON(state.display);
-    this.playing = state.playing;
     this.time = state.time;
     this.timeScale = state.timeScale;
     this.setCurrentMode(state.currentMode);

style.css

@@ -87,7 +87,7 @@ div.lhg-tool div.lhg-wide {
   flex-direction: row;
 }
 
-div.lhg-tool button, div.lhg-tool input, div.lhg-tool a {
+div.lhg-tool button, div.lhg-tool input {
   font-family: monospace;
   font-size: 10pt;
   background-color: #333;
@@ -106,6 +106,11 @@ div.lhg-tool button, div.lhg-tool input, div.lhg-tool a {
   box-sizing: border-box;
 }
 
+div.lhg-tool button a {
+  color: #5f5;
+  text-decoration: none;
+}
+
 div.lhg-tool button:hover, div.lhg-tool input:hover {
   background-color: #444;
 }

system.js

@@ -1,9 +1,8 @@
 import {EVENT_OBJECT_CREATE, EVENT_OBJECT_MERGE, OBJECT_HISTORY_SIZE} from './config.js';
 import {MassObject} from './object.js';
 import {
-  add, copy, cross, degrees,
-  direction, div, magnitude, mult,
-  square, sub, weightedAvg, zero
+  add, copy, cross, degrees, direction, div,
+  dot, magnitude, mult, square, sub, weightedAvg, zero
 } from './vector.js';
 
 export class System {
@@ -93,12 +92,11 @@ export class System {
     if (this.sim.playing) {
       // Predict positions (Velocity verlet method)
       this.forEachObject(obj => {
-        obj.currentAcceleration = {...obj.acceleration};
+        obj.currentAcceleration = copy(obj.acceleration);
 
-        // If this object is being created/selected, clamp its position
-        if (obj.id === this.getSelectedOrCreating()?.id) {
-          return;
-        }
+        // If this object is being created/selected, we're not going to let it move...
+        // but we can calculate the work being done by holding it in place.
+        obj.currentPosition = copy(obj.position);
 
         obj.position = add(obj.position, mult(
           elapsedTime,
@@ -162,10 +160,21 @@ export class System {
 
       // Predict velocities
       this.forEachObject(obj => {
-        const acceleration = {...obj.acceleration};
+        const acceleration = copy(obj.acceleration);
         obj.acceleration = div(add(obj.currentAcceleration, acceleration), 2);
         obj.velocity = add(obj.velocity, mult(obj.acceleration, elapsedTime));
 
+
+        // If the user is positioning this object, we'll leave its position unchanged;
+        // but let's compute how much work we're doing to accomplish it!
+        if (obj.id === this.getSelectedOrCreating()?.id) {
+          const delta = sub(obj.currentPosition, obj.position);
+          const netForce = mult(obj.acceleration, obj.mass);
+          const work = dot(netForce, delta);
+          console.log('work', work);
+          obj.position = obj.currentPosition;
+        }
+
         // Append to object history
         obj.history.push({position: {...obj.position}});
 

tool/state.js

@@ -5,7 +5,7 @@ import {Tool} from '../tool.js';
 export class StateTool extends Tool {
   stored = [];
 
-  setContainer(container) {
+  async setContainer(container) {
     super.setContainer(container);
 
     const buttons = document.createElement('div');
@@ -31,20 +31,45 @@ export class StateTool extends Tool {
     });
 
     // Check url query parameter, and load specified state if found
+    await this.fromUrl();
+  }
+
+  async toUrl(state) {
+    const stateText = JSON.stringify(state);
+    const digest = await hash(stateText);
+    const rawUrl = `./?state=${stateText}&digest=${digest}`;
+    const url = encodeURI(rawUrl);
+    return {url, digest};
+  }
+
+  async fromUrl() {
     const paramsString = window.location.search;
     const searchParams = new URLSearchParams(paramsString);
-    const stateEnc = searchParams.get("state"); // a
+    const stateEnc = searchParams.get("state");
+    const rxDigest = searchParams.get("digest");
     if (stateEnc) {
       const stateText = decodeURI(stateEnc);
+      console.log('decoded state text', stateText);
       const state = JSON.parse(stateText);
+      const digest = await hash(stateText);
+      if (digest !== rxDigest) {
+        throw new Error('state query parameter does not match digest query parameter');
+      }
       // Tools in this system can be very powerful
+      this.sim.pause();
       this.sim.fromJSON(state);
     }
   }
 
   getStateDescription(state) {
-    const d = new Date(state.dateSaved);
-    return `${d.toLocaleDateString()} ${d.toLocaleTimeString()}`;
+    const date = new Date(state.dateSaved);
+    const Y = date.getFullYear().toString();
+    const M = (date.getMonth() + 1).toString().padStart(2, '0');
+    const D = date.getDate().toString().padStart(2, '0');
+    const h = date.getHours().toString().padStart(2, '0');
+    const m = date.getMinutes().toString().padStart(2, '0');
+    const s = date.getSeconds().toString().padStart(2, '0');
+    return `${Y}-${M}-${D} ${h}:${m}:${s}`;
   }
 
   async createItem(state) {
@@ -60,34 +85,24 @@ export class StateTool extends Tool {
 
     const load = document.createElement('button');
     load.style.flex = '1';
-    load.innerHTML = 'Load';
 
     const link = document.createElement('a');
     const {url, digest} = await this.toUrl(state);
-    link.classList.add(TOOL_INFO_CLASSNAME);
-    link.classList.add(WIDE_CLASSNAME);
     link.href = url;
-    link.innerHTML = digest.slice(0, 6);
+    link.innerHTML = digest.slice(0, 5);
+
+    load.appendChild(link);
 
     item.appendChild(description);
     item.appendChild(load);
-    item.appendChild(link);
 
-    load.addEventListener('click', () => {
+    load.addEventListener('click', (e) => {
+      e.preventDefault();
       // Tools in this system can wield great power
+      this.sim.pause();
       this.sim.fromJSON(state);
     });
 
     return item;
   }
-
-  async toUrl(state) {
-    const stateText = JSON.stringify(state);
-    // const stateB64 = window.btoa(stateText);
-    const rawUrl  = `./?state=${stateText}`;
-    const url = encodeURI(rawUrl);
-    const digest = await hash(stateText);
-    return {url, digest};
-  }
-
 }

tool/zoom.js

@@ -85,7 +85,9 @@ export class Zoom extends Tool {
 
     zeroVelocity.addEventListener('click', () => {
       // Determine center of mass and average momentum
-      const {totalMass, netMomentum} = this.sim.system.computeSystemCenter();
+      const objects = this.sim.select.selectedGroup;
+      const {netMomentum} = this.sim.system.computeSystemCenter(objects);
+      const {totalMass} = this.sim.system.computeSystemCenter();
       const netVelocity = {
         x: netMomentum.x / totalMass,
         y: netMomentum.y / totalMass,